Finding The Aromic Mass: A Step-By-Step Guide

Aromatic compounds are a subset of organic molecules that are unusually stable. To determine if a molecule is aromatic, it must meet four conditions:

1. The molecule must be cyclic.

2. Every atom in the ring must be conjugated.

3. The molecule must have [4n+2] pi electrons.

4. The molecule must be flat.

The molecule must be cyclic

A cyclic compound is a compound in which one or more series of atoms in the compound are connected to form a ring. Rings can vary in size from three to many atoms. For example, cycloalkanes are the simplest carbocycles, including cyclopropane, cyclobutane, cyclopentane, and cyclohexane.

In organic chemistry, the term "aromaticity" is used to describe a cyclic (ring-shaped), planar (flat) molecule that exhibits unusual stability compared to other geometric or connective arrangements of the same set of atoms. Aromatic compounds are more stable than theoretically predicted using hydrogenation data of simple alkenes; the additional stability is due to the delocalized cloud of electrons, called resonance energy.

Aromatic molecules undergo substitution rather than addition reactions. They have an extremely high resonance energy (36 kcal/mol for benzene) and delocalized pi-electrons.

Aromaticity is such a stabilizing property that a molecule that has [4n+2] pi electrons will also be flat.

Cyclic compounds are the largest majority of all molecules involved in the biochemistry, structure, and function of living organisms, and in man-made molecules (e.g. drugs, herbicides, etc.).

Every atom in the ring must be conjugated

Aromaticity is a property of some unusually stable organic molecules such as benzene. Aromatic molecules have an extremely high resonance energy, undergo substitution rather than addition reactions, and have delocalized pi-electrons.

There are four key rules for aromaticity. The first is that the molecule must be cyclic. The second is that every atom in the ring must be conjugated. The third is that the molecule must have [4n+2] pi electrons. The fourth is that the molecule must be flat.

The second rule states that every atom in the ring must be conjugated. In other words, there must be a continuous ring of p-orbitals around the ring that build up into a larger cyclic "pi system". Each atom around the ring must be capable of conjugation with each other. This means that every atom in the ring must have an available p orbital and be able to participate in resonance.

The "available p orbital" condition applies not just to atoms that are part of a pi bond, but also atoms bearing a lone pair, a radical, or an empty p orbital (e.g. carbocations). The key thing that “kills” conjugation is an sp3 hybridized atom with four bonds to atoms. Such an atom cannot participate in resonance.

The molecule must have 4n+2 pi electrons

Aromaticity is a property of certain molecules that gives them enhanced stability. Aromatic molecules are cyclic and planar, with a ring of resonance bonds that exhibit greater stability than other connective or geometric arrangements within the same kind of atoms. Aromatic molecules are also highly reactive and are essential in the production of many chemicals and polymers.

The German chemist and physicist Erich Hückel proposed a theory in 1931 to determine if a molecule would have aromatic properties. This theory, known as Hückel's Rule, states that a molecule must have 4n+2 π electrons to be considered aromatic. In this rule, n can be 0 or any positive integer (1, 2, 3, etc.).

Hückel's Molecular Orbital Theory explains that a compound is particularly stable if all its bonding molecular orbitals are filled with paired electrons. Aromatic compounds follow this theory, as they have 2 electrons filling the lowest energy molecular orbital, and 4 electrons filling each subsequent energy level. This leaves all bonding orbitals filled and no anti-bonding orbitals occupied, resulting in a total of 4n+2 π electrons.

To apply Hückel's Rule, count the number of π electrons in the molecule and set this number equal to 4n+2. Then, solve for n. If n is 0 or a positive integer, the rule is met. For example, benzene has 6 π electrons:

4n + 2 = 6

4n = 4

N = 1

Since n equals 1, a positive integer, benzene follows Hückel's Rule and is considered aromatic.

It is important to note that not all molecules with 4n+2 π electrons are aromatic. The molecule must also be cyclic, planar, and fully conjugated (with p orbitals at every atom in the ring) to be considered aromatic.

The molecule must be flat

The fourth condition for aromaticity is that the molecule must be flat (planar).

Aromaticity is such a stabilising property (worth 20-36 kcal/mol) that generally a molecule that has [4n+2] pi electrons will also be flat. Give a molecule a large enough potential energy well, and it will fall into it eventually.

It’s a bit like the punchline to the old (crude) joke about why dogs adopt a certain energetically favourable conformation: “Because they can”.

As with certain vertebrates, the only thing that prevents a molecule that fulfils the first three conditions from being flat is if the flat conformation is incredibly strained. One example in this category is the molecule known as [10]-annulene, an isomer of which is drawn below left. In the trans, cis, trans, cis, cis isomer, the molecule is cyclic, conjugated, and has 10 pi electrons, but the two marked hydrogens bump into one another when attempting to adopt a flat conformation.

The molecule is prevented from adopting planarity due to this punitive Van Der Waals strain, and is therefore not aromatic.

Interestingly, if the hydrogens are removed and replaced with a bridging CH2 group, the strain is relieved and the pi bonds can adopt a planar conformation. The molecule below right shows the expected properties of an aromatic molecule.

The Huckel Rule

Hückel's Rule is a theory proposed by German chemist and physicist Erich Hückel in 1931. It is used to determine whether a planar ring molecule will have aromatic properties. The rule states that a molecule is considered aromatic if it has 4n+2 π-electrons, where n is a non-negative integer. This can be any integer with a positive value, including zero.

The rule can be applied to molecules containing other atoms such as nitrogen or oxygen. For example, pyridine has a ring structure similar to benzene, but with one -CH- group replaced by a nitrogen atom with no hydrogen. There are still six π electrons, and the pyridine molecule is also aromatic and known for its stability.

To apply the 4n+2 rule, first count the number of π electrons in the molecule. Then, set this number equal to 4n+2 and solve for n. If n is 0 or any positive integer (1, 2, 3,...), the rule has been met. For example, benzene has six π electrons:

4n + 2 = 6

4n = 4

N = 1

For benzene, we find that n=1, which is a positive integer, so the rule is met.

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